In the field of induction motors, a cast squirrelcage rotor, which includes a laminated core member made by stacking magnetic laminations, and a conductor portion integrally formed with the laminated core member by pouring molten metal, such as molten aluminum or copper, into a plurality of slots provided in the laminated core member, is well known. Such a cast squirrel-cage rotor has the advantage of good productivity, and thus is frequently used in induction motors as, e.g., drives for machine tools.
One example of a conventional casting process for a squirrel-cage rotor is described below with reference to FIG. 6a. In this casting process, a plurality of laminations forming a laminated core member 1, are held by a holding member 2 in a laminated state, and then are placed together with the holding member 2 in a mold 3. Thereby, axially extending slots 4 are formed at an outer periphery of the laminated core member 1. Also, balancing ring members 5 are generally arranged for ensuring a rotor balance at respective axial ends of the laminated core member 1. The outer diameters of the balancing ring members 5 are smaller than the outer diameter of the laminated core member 1, whereby annular cavities 6 for communicating the cavities of the slots 4 with each other are formed at respective axial ends of the laminated core member 1 placed in the mold 3.
The laminated core member 1 accommodated in the mold 3 is positioned in such a manner that, e.g., the axis thereof is vertically oriented. Then, molten metal is poured, at a predetermined pressure, into the mold cavity through a gate 8 from a molten-metal pouring opening 7 located beneath the laminated core member 1. In this case, the molten metal flows in a direction opposite to gravity and successively fills one (lower) annular cavity 6, slots 4, and the other (upper) annular cavity 6, whereby secondary conductors in slots 4 and end rings in annular cavities 6 are respectively formed integrally with the laminated core member 1. The mold 3 is provided with a plurality of air-vents 9 for communicating the other (upper) annular cavity 6 with the outside of the mold.
The holding member 2 for holding the laminated core member 1 includes a sleeve element 10 extending through a shaft bore of the laminated core member 1, and a lid element 11 joined to one end of the sleeve element 10. The sleeve element 10 and the lid element 11 are provided with radially extending flanges 12 and 13, respectively. Therefore, when the sleeve element 10 and the lid element 11 are firmly fastened together by a fastening means 14 such as a bolt, the flanges 12, 13 hold the laminated core member 1 in a laminated state through the balancing ring members 5.
In the conventional casting process, the distance between the flanges of the holding member for holding the laminated core member corresponds to the axial length of the laminated core member and, if necessary, a pair of balancing ring members, as mentioned above. Therefore, in the case of forming squirrel-cage rotors having different axial lengths by casting, it has been necessary to prepare different holding members and different molds corresponding to the axial lengths of the rotors. FIG. 6b shows, by way of example, a holding member 2' and a mold 3' for carrying out a casting process to a laminated core member 1' which has an axial length shorter than that of the laminated core member 1 of FIG. 6a.
Generally, the output of an induction motor is proportional to the value of the resistance of a secondary conductor, so that it is possible to form various kinds of induction motors having different outputs, by modifying the axial length of the squirrel-cage rotor. However, it is not preferred to prepare many kinds of holding members and molds so as to be able to form rotors corresponding to all desired outputs, because this increases the production cost by increasing the cost of the equipment. Thus in the past, in order to produce induction motors having different outputs, rotors having same axial length were used and the outputs were changed by modifying the electric characteristics.
When the output of an induction motor is changed by merely modifying the electrical characteristics, problems arise in which, e.g., the weight and the output of the core are not balanced according to rotor size, and the acceleration time is increased. Recently, in, e.g., an induction motor for driving a spindle of a machine tool, good acceleration properties have been desired to reduce processing time, and thus it has become necessary to use a rotor having an optimum size corresponding to the required output. However, as mentioned above, when many kinds of laminated core members corresponding to various outputs are produced, the problem of an increase in the production cost due to the cost of equipment must be overcome.